This invention relates generally to subsurface well apparatus and more specifically, to methods and apparatus for perforating subsurface earth formations utilizing a tubing conveyed perforating apparatus.
It has become common practice in the completion of oil and gas wells to perforate the well casings and the surrounding formations to bring a well into production. One method of providing this capability has a perforating apparatus attached to the end of a tubing string which is lowered and set in place when the perforating apparatus is opposite the formation to be produced. The perforating apparatus may then be detonated and the well placed into production through the tubing strings.
The systems for firing the perforating apparatus have typically been either an electrical firing system or a non-electric percussion firing system. Neither method has been entirely satisfactory in the past since electrical firing systems require care in connection and running because these systems can be activated from stray electrical currents. In addition, electrical connections can be short-circuited by moisture. Percussion firing systems commonly have some primary explosives in the perforating apparatus while it is affixed to the tubing and lowered into position. As a result of the deficiencies of these firing systems, accidental and premature firings are a possibility. Further, in the event of a malfunction, making removal of the perforating apparatus necessary, the chance of accidental ignition of the perforating apparatus could prove dangerous to personnel.
These and other disadvantages are overcome with the present invention by providing a method and an apparatus for perforating well casing and the surrounding earth formations using a primary percussion firing system which is installed in the perforating assembly only after the perforating apparatus has been set and additionally by providing for removal of the percussion firing system in case of malfunction and installation of another percussion firing system or a secondary electrical firing system which likewise can be removed in case of malfunction.